This invention relates to an automatic sewing machine for carrying out sewing operation according to stitch instructions which are read out or extracted in a predetermined sequence from a memory and include position data designating a relative position between a needle and a workpiece, and more particularly to the drive motor speed control of the machine in the sewing operation.
It has been conventional practice in this type of sewing machines that a permissible drive speed, i.e., the maximum permissible drive speed of the drive motor is determined according to the kind of sewing operation specified to an individual machine. In other words, a machine which sewing operations including a fairly large relative positional variation or relative movement between the needle and the workpiece, is allotted from the outset a comparatively low maximum permissible speed corresponding to the larger relative movement allowed. This maximum permissible speed naturally regulates all kinds of other sewing operations which the automatic sewing machine can perform. A specific speed control for the drive motor for the automatic sewing machine is designated or determined according to the above-mentioned predetermined maximum permissible speed allotted thereto.
In this sort of automatic sewing machine, even when a sewing operation can be carried out with comparatively small relative movements, the drive motor was in fact obliged to be limited to a maximum permissible speed of a comparatively low level irrespective of its capability of driving at a higher speed from the standpoint of the machine's construction and the performance of the drive motor. There thus arose a problem of using the machine at a performance level much lower than the true variable performance level when the sewing operation included only a smaller relative movement amount between the needle and the workpiece, which degraded the efficiency of the sewing operation uselessly. In order to eliminate this disadvantage, U.S. Pat. No. 4,108,093 discloses means for installing a speed control circuit capable of regulating the speed of the drive motor so as to raise it up to the maximum permissible speed corresponding to a sewing operation carried out by the amount of the least relative movement among various types of sewing operations specified to an automatic sewing machine, and letting a memory store predetermined maximum permissible speeds in accordance with each sewing operation mode. In this cited reference, selection and change of the maximum permissible speed in response to the selection of each stitch pattern became possible, because various maximum permissible speeds can be memorized permanently or fixed according to the modes of the stitching patterns used.
In this type of technology the respective maximum permissible speed is stored permanently in the memory according to each sewing operation. When this data concerning the maximum permissible speed is stored in the memory by a programmer, the variation of the relative position according to each stitch instruction must be individually checked before the maximum permissible speed can be determined by the programmer.
This prior technology is still disadvantageous as stated herein. For example, these disadvantages include that it is a fairly large burden on the programmer. In the field of industrial machines, more than a thousand stitches are usually formed in a stitch pattern. To check all of the variations of the relative positions under the control of each stitch instruction, this entails a substantial risk of mis-storing a maximum permissible speed.